Toward understanding the relationship between insulation recovery and micro structure in water tree degraded XLPE cables

In this paper, to understand the insulation recovery phenomenon of water-tree cables when an applied voltage is removed, the insulation recovery mechanism of the water tree cable is analyzed. A water-needle electrode is used in an accelerated water-tree aging experiment for XLPE (Cross Linked Polyethylene) cables. The dielectric loss factor (tanδ) increases with aging time, but it gradually returns to a lower level once the applied voltage is removed for some time, which means that the insulation recovery of the water tree cables possibly occurs during the process. Depending on the geometric size by micro observations of water-tree slices, a watertree model is constructed for electric field simulation, which includes a series of water-filled micro voids and interconnected channels. According to analysis of the electric field force and the mechanical properties of water tree region, the force generated by the elastic deformation of the molecular chains results in the shrinking of the channels. The water is gradually squeezed out of the channels, which causes the gradual decrease of conductivity and relative dielectric constant in channels. According to the computed results of tanδ at different states of water tree, the reduction of tanδ can be mainly caused by the decrease of conductivity and relative dielectric constant in channels. The time needed for insulation recovery may be determined by the internal electric field in the water tree, the mechanical relaxation time of XLPE, and the diffusion of water.